Compact fluid heater

ABSTRACT

THE FIN SURFACES IN ANY PLANE ARE SUBDIVIDED INTO CONCENTRIC RINGS OR CYLINDERS WHOSE INNER PERIPHERIES ARE EACH ATTACHED TO THE OUTER EXPOSED SURFACES OF A CIRCULAR ARRAY OF PIPES, SO THAT THE CONCENTRIC ARRAYS OF PIPES ARE MECHANICALLY SEPARATED. A COMPACT CONFIGURATION FOR HEATING OF A LIQUID, BOILING OF A LIQUID AND SUPERHEATING THE VAPOR OF A LIQUID. A CYLINDRICAL ARRANGEMENT IS PROVIDED IN WHICH PRODUCTS OF COMBUSTION ARE INTRODUCED FROM THE PERIPHERY OF THE CONFIGURATION TOWARDS THE AXIS THEREOF IN A RADIALLY INWARD DIRECTION WHILE THE FLUID TO BE HEATED FOLLOWS A LABYRINTHINE PATH FROM THE AXIAL REGION TOWARDS THE PERIPHERAL REGION. THE LABYRINTHINE PATH CONSISTS PREDOMINANTLY OF LENGTHS OF PIPE OF SMALL DIAMETER ARRANGED AXIALLY IN A CYLINDRICAL ARRAY IN UP TO THREE COAXIAL ZONES. IN THE INNERMOST ZONE THE FLUID IS LIQUID. IN THE OUTERMOST ZONE THE FLUID IS IN THE VAPOR STATE AND THE INTERMEDIATE ZONE CONSTITUTES THE BOILER. HEAT TRANSFER SURFACES ARE DIFFERENT IN EACH ZONE. NOTHING IS ADDED TO THE PIPES IN THE VAPOR ZONE. THE PIPES IN THE BOILER ARE MECHANICALLY CONNECTED TOGETHER BY FLAT PLATE FINS OR OTHER EXTENDED SURFACES EXTENDING IN THE RADIAL DIRECTION SO AS TO PROVIDE MINIMUM INTERFERENCE WITH FLOW OF THE PRODUCTS OF COMBUSTION. SIMILAR EXTENDED SURFACES ARE USED IN THE INNERMOST ZONE, EXCEPT THAT IN THIS ZONE

Inventor Jan P. R005 ABSTRACT: A compact configuration for heating of a liquid. 3l-A Park Drive. Woburn. Mass. 01801 boiling of a liquid and superheating the vapor of a liquid. A

Appl. No. 854,965 cylindrical arrangement is provided in which products of com- Filed Sept. 3, 1969 bustion are introduced from the periphery of the configura- [45l P te J n 2 197! tion towards the axis thereof in a radially inward direction while the fluid to be heated follows a labyrinthine path from the axial region towards the peripheral region. The

labyrinthine path consists predominantly of lengths of pipe of COMPACT FLUID HEATER small diameter arranged axially in a cylindrical array in up to 5 Claims 3 Drawing g three coaxial zones. in the innermost zone the fluid is liquid. In

the outermost zone the fluid is in the vapor state and the intermediate zone constitutes the boiler. Heat transfer surfaces are different in each zone. Nothing is added to the pipes in the vapor zone. The pipes in the boiler are mechanically connected together by flat plate fins or other extended surfaces United States Patent wZON OZCHMI 0503 t t 4 j HEADERS n A H 1:1

I w W HEADERS extending in the radial direction so as to provide minimum interference with flow of the products of combustion. Similar extended surfaces are used in the innermost zone, except that in this zone the tin surfaces in any plane are subdivided into concentric rings or cylinders whose inner peripheries are each attached to the outer exposed surfaces of a circular array of pipes, so that the concentric arrays of pipes are mechanically separated.

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HEIADEIRS INVENTOR JAN P. ROOS Bab-53 i' l s ATTORNEYS GAS OIL VAPOR I INLET PATENTED JUH28 I97:

SHEET 2 BF 3 INVENTQR JAN P R005 ATTORNEYS PATENTEUJUN28197: 3587.532

SHEET 3 OF 3 FIG. 3

INVENTOR JAN P. ROOS ATTORNEYS VAPOR- EXIT COMPACT FLUID HEATER BACKGROUND OF THE INVENTION 1. Field ofthe invention The invention relates to liquid heaters and vaporizers.

BACKGROUND OF THE INVENTION 2. Description ofthe Prior Art Conventional boilers are well known and need not be described in any detail herein. Reference may be made to the following US. Pat. Nos: 3,384,052; 3,352,289; 3,l2l,420; 2,898,892; 1,998,329; l,903,807;and 1,378,307.

SUMMARY The invention provides a compact configuration for heating of a liquid, boiling ofa liquid and superheating the vapor ofa liquid.

By properarrangement of heat transfer surfaces, it is possible to achieve high heat transfer capacities of the order of B.t.u./hr ft with a gas or fuel-air fired furnace system produced hot flue gas to heat the coolant medium.

The compactness of the compact fluid heater is primarily a result of the choice of low hydraulic diameters and of fluid flow directions which are compatible with the chosen configuration in the sense that they harmonize with the natural laws that predict that gases decrease in volume upon cooling and that a fluid, when heated to a vapor and successively to a superheater vapor, increases in volume whenthe pressures for both are maintained at essentially the same level (i.e. within :5 percent). The general configuration of the compact fluid heater is that ofa cylinder with a concentric hollow cylindrical core. The hot flue gases are generated by burning of a gas or fuel at the outside cylindrical surface of the compact fluid heater. By forced convection the hot flue gases are drawn radially through the annular cylinder, that is the compact fluid heater proper, into the hollow core, which is the stack for the exiting flue gas. The liquid enters the compact fluid heat at the inside surface of the annular cylinder and progresses while changing state from liquid to a vapor to the outside cylindrical surface in a cross-counter flow fashion.

A typical feature of the compact fluid heater is that the density of the heating furnace gas is low as compared to the density of the heated liquid-vapor medium. it is also a feature of this compact fluid heater that the operating pressure of the liquidvapor medium is high and near the critical pressure of this medium, be it below or above the critical pressure. The practical result of this is that the density variation from the liquid state to the vapor state of this heated medium is small, allowing the flow cross section variation from coolant exit to coolant inlet to remain small (i.e. less than a factor of 30) and'also allowing the two phase flow pressure drop in a boiling section to remain small (i.e. less than a factor of times the equivalent liquid pressure drop).

The compact fluid heater can employ both plain heat transfer surfaces as well as extended fin-type heat transfer surfaces. Plain heat transfer surfaces can be employed at the highest flue gas temperature side of the compact fluid heater, to keep the temperatures of the metal in that region'close to the exiting coolant temperature. Extended fin-type heat transfer surfaces are employed where the flue gases are cooled sufficiently to not cause excessively high temperatures in the extended fin material nor in the metal containing the coolant medium.

Connected with the high density ratio of coolant to flue gas is a high ratio of heat transfer coefficient between coolant and flue gas. As a consequence the extended heat transfer surfaces are formed on the flue gas side of the fluid-conduit surfaces except possibly at the inlet of the hottest flue gas into the compact fluid heater as noted above. Another important feature of the chosen compact fluid heater configuration and its fluid flow directions is that more of plain heat transfer surface area is now available to the hot furnaces gases than at any other major boundary surface of the compact fluid heater. Therefore the large quantity of plain heat transfer surfaces emto the compactness of the fluid heater by not requiring a large portion ofthe furnace gas heat transfer path.

This compact fluid heater is basically different from conventional boilers in that radiant heat transfer from a bright furnace flame zone to the metal heat transfer surfaces is not essential nor required and plays a negligible role if any role at all. All heat transfer is obtained primarily by forced convection of all fluid involved. Natural convection or draft" is not desired. As a consequence the flame can be an invisible flame, as is typical for a complete combustion, without the bright radiant energy emitted by uncombusted particles such as carbon or fly ash in a conventional furnace hearth. The use of a complete combustion flame that gives almost no solid deposits on the heat transfer surfaces is essential for reliable operation of this compact fluid heater because of the small hydraulic diameters of this design; as low as of the order of one-tenth of an inch.

The design of the compact fluid heater eliminates undue thermal stresses by mechanically uncoupling the low temperature sections from high temperature sections in the heater. To illustrate this: The first row of liquid heating tubes with their extended surfaces are not connected to the adjacent extended surfaces of the second row of liquid heating tubes, and so on, except by the return bends. Hence increasing temperature for the consecutive tube rows and the associated thermal expansion in the length direction of the tubing can not create thermal stresses in the extended surfaces which conventionally would connect the tube rows. Hence high cyclic stresses that would appear because of noncontinuous operation at high heat transfer levels are dramatically reduced and will not lead to failures at the extended fin and tube interface, that (could) impede heat transfer and then subject the extended fin to high temperature and destruction as well as decrease the overall system heat transfer coefficient.

The boiling section of the compact fluid heater will however remain at nearly the same temperature because of the boiling phenomenon itself. A large amount of heat is added to the liquid to turn it into steam at constant temperature, which is the saturation temperature belonging to the pressure at which the boiler operates. Typically this temperature is 636'F. for 2,000 p.s.i.a. water. Because of the constant boiler temperature all tube rows in the boiling section can have interconnected extended fln surfaces therefore without danger of introducing thermal stresses if the manufacturing process is simplified by doing so. The tube rows that make up the superheating section of the compact fluid heater are typically not provided with extended surfaces and hence are free from the above thermal stress problems as long as burning intensity is nearly constant all around the outside of the burner so as to keep the temperature distribution of each heater and tube row symmetric with respect to the centerline of the compact heat exchanger.

A heat transfer analysis of the compact heat exchanger was performed as a stepwise numerical calculation for a 3,400 F. flue gas and l',050 F., 2000 p.s.i.a. boiling water system. The following table compares values of an average commercial boiler with a compact boiler of above specifications, both capable of handling 6-million B.t.u. per hourv Commercial Compact Boiler burner volume, ft. 285 2% Dry weight \\'/0 burner, lb. 9, 000

Weight of water fill, 1b. 7, 500 4 Heat transfer surfacea. water side, it. 820 17 b. fire side, ft. 750 167 heat available as process heat due to its resulting decrease of enthalpy because of its partial conversion into mechanical power.

It is noted that the compact fluid heater generates high pressure, high temperature steam with the same flame temperature as the conventional boiler. It is a feature of the compact fluid heater that steam or superheated fluid distribution can be accomplished with smaller pipe diameters than is the case for lower pressure steam for the following reasons: A rate of steam use going through a steam pipe is proportional to steam density, steam velocity and pipe cross section or pipe diameter squared. For constant use the pipe diameter is therefore proportional to the inverse square root of steam density times steam velocity. The high pressure steam has a high density that hence permits decreasing the pipe diameter. The high density steam allows substantial larger pressure drops to occur in the steam distribution piping for low pressure steam applications than in conventional process steam distribution systems. The higher obtainable steam flow velocities as a result again permit the distribution pipe diameter to be smaller. An essential feature or advantage of the compact fluid heater therefore is the possibility of a compact steam distribution system that saves space and installation cost.

A characteristic of the invention is that it can produce steam at high pressure and temperature. The pressure and temperature at which the steam is produced is much higher than now normal for packaged boilers with exception of large steam power plants. As a result, as shown in the above paragraph, the invention permits new steam distribution possibilities. Utilizing the principles of the invention, a central power pack delivering steam at 2,000 p.s.i. and 1,000 F. could be provided and could be connected to various pressure regulators so that various applications could be furnished with steam at lower pressures. For example, from the central power pack steam could be provided for piston engines at a pressure of 600 p.s.i. and steam could be provided for heating purposes at 150 p.s.i. The steam provided for heating could be the same steam previously utilized for driving the piston engines.

The principles of the invention require the use of small diameter piping. As noted, the diameter of the pipes used in the compact fluid heater of the invention may be as small as one-eighth of an inch. The use of smaller diameter piping permits the use of light piping. The compactness furnished by the invention makes a power pack furnishing steam at 2,000 p.s.i. both feasible and economical. The present cost for packaged boilers is approximately $l.50/l,000 B.t.u./hour. Projected costs for a compact fluid heater in accordance with the invention and capable of furnishing 6,000,000 B.t.u./hour and with auxiliary equipment will not exceed $9,000.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may best be understood from the followiing detailed description thereof having reference to the accompanying drawings in which:

FIG. 1 is a vertical central section of a compact fluid heater constructed in accordance with the invention;

FIG. 2 is a section taken along the line 24 of FIG. 1; and

FIG. 3 is a detail ofa portion of the apparatus of FIG. 1.

Referring to the drawings, the compact fluid heater of the invention comprises a generally cylindrical housing 1 within which the fluid to be heated flows through a conduit 2 in a labyrinthine path. The conduit for the fluid to be heated is composed of a multiplicity of tubes of small bore. For example, the tube diameter may be as small as one-eighth of an inch. These tubes are arranged parallel to one another and to the axis of the overall heater in a concentric series of annular arrays. They are connected to one another at the ends of the device by headers or return bends in a manner which differs depending upon the particular section in which the particular tubes are located. The total multiplicity of tubes is divided into three concentric zones. The fluid is introduced as a liquid into the innermost tubes of the innermost zone and the flow of the fluid is through the tubes progressing radially outward from layer to layer. Within each zone the tubes are arranged in concentric layers. In the intermediate zone, the fluid reaches its boiling point and so this intermediate zone may conveniently be referred to as the boiler zone. The fluid enters the outermost zone in the vapor state and is removed at maximum temperature from the periphery of the device. The tubes are arranged in a plurality of parallel paths one of which is traced in FIG. 3. Following one such path as shown in FIG. 3 liquid is introduced into the annular header 3 to which the tubes 4 of the innermost annular array are connected to one extremity thereof. In a particular design with aforementioned specifications the liquid travels from the annular header 3 in 36 parallel paths to the upper extremities of the tubes 4 of the innermost annular array, where elbows 5 redirect the liquid respectively into the tubes 6 of the second innermost annular array. The liquid travels from the upper extremities of the tubes 6 in 36 parallel paths to the lower extremities thereof, where elbows 7 redirect the liquid into the tubes 8 of the third innermost annular array. The liquid then travels from the lower extremities of the tubes 8 in 36 parallel paths to the upper extremities thereof, where elbows? redirect the liquid into the tubes 10 of the fourth innermost annular array. The liquid then travels from the upper extremities of the tubes 10 in 36 parallel paths to the lower extremities thereof, which are connected to a lower annular boiler header 11. The fluid does not travel a serpentine path through the boiler, but rather the 180 straight lengths oftube l2, l3, 14 in the boiler are connected in parallel, the fluid flowing therethrough from the lower annular boiler header 11 to an upper annular boiler header 15, which the fluid enters in vapor phase. The upper extremities of the tubes 16 of the innermost annular array in the outermost zone are connected to the upper annular boiler header 15, and the vapor travels from the upper annular boiler header 15 in I60 parallel paths to the lower extremities of the tubes I6 of the innermost annular array in the outennost zone, where elbows l7 redirect the vapor respectively into the tubes 19 of the intermediate annular array in the outermost zone. The innermost annular array and the intermediate annular array of the outermost zone are each a double array made up of two concentric but staggered single arrays, as shown in FIG. 2. The vapor then travels from the lower extremities of the tubes 18 in parallel paths to the upper extremities thereof, which are connected to an upper vapor header 19. The fluid then flows from the upper vapor header 19 to a lower vapor header 20 through 288 straight lengths of tube 21, 22 forming the two outermost annular arrays The vapor issues at high temperature from the lower vapor header 20,

The fluid is introduced at relatively high pressure such as 2,000 lbs. per square inch. For this purpose, a liquid pump 23 is provided in the input conduit 24 leading to the annular header 3.

The fluid is heated by hot gas which is produced by burning an appropriate fuel about the periphery of the device. For example, oil vapor may be introduced through a pipeline 25 into an appropriate chamber or flame region 26 located on the periphery of the device, where it is mixed with air introduced through an air intake conduit 27 and burned. The products of combustion travel from the periphery of the device to the tubular center which constitutes the flue. From the flue the gases which are still at relatively elevated temperature may pass through a heat exchanger (not shown) in order to transmit their heat to the incoming air. In a representative example, the hot flue gases may enter the periphery at about 3,400 F. and may exit through the central exhaust at about 300 F.

Forced convection of the gas flow is necessary to the invention, since the object of the invention is compactness. A natural draft would require a huge chimney. Accordingly, forced convection may be provided by any conventional means, such as an air blower as shown schematically at 28 in FIG. I. Alternatively, a pressurized reservoir may be employed to provide forced convection, as in the case of space environment where oxygen might be delivered by the reservoir.

An important part of the invention is the construction ofthe heat exchange surface between the fluid to be heated and the hot flue gases. The inner surfaces of the tubes provide the plain heat transfer surface to all phases of the heated fluid. In the outer vapor phase zone the gas side heat exchange surface is simply the outer surface of the small bore pipes. Throughout the boiler zone fins are secured to the tubes. The boiler zone is the region where a larger temperature difference in the metal is allowed because of the larger difference between safe operating metal temperature and the fluid temperature. As a result more heat transfer per tube area is permitted. Due to the relatively small heat transfer coefficient between the gas and the metal, this can only be realized by increasing the surface area on the gas side by adding extended surfaces as mentioned above. Accordingly, fins are secured to the tubes in the boiler zone with their surfaces arranged perpendicular to the axis in order to provide reduced interference with the flow of the hot gases. The temperature within the boiler zone is substantially the same throughout the boiler zone since the effect ofthe heat being added is to change the phase of the fluid and not to raise the temperature. Consequently, mechanical problems of thermal expansion are at a minimum and each fin may extend across the entire boiler zone and be secured to all tubes in its plane, so as to provide a mechanically rigid structure. In the liquid phase zone however, the temperature is increasing from the innermost layer to the outermost layer, and so the fins in this zone are subdivided into concentric rings each of which is secured only to the outer peripheral surfaces of a single array of tubes. Alternatively, the extended surfaces in the liquid phase zone may comprise mesh fins connected, as in the case of the concentric rings, only to the outer periphery of each array and not to the inner periphery of the next array, in order to provide room for thermal expansion.

The steam has a relatively high heat capacity just after it has been vaporized: the heat capacity of saturated steam at high pressure at this temperature is about 3, compared with a value of about 0.6 at higher temperatures. Consequently, in the innermost array of the vapor zone the temperature rise of the steam continues to be relatively low, and the pipesl6 of this array may be affixed to the fins of the boiler zone. In the outermost array of the vapor zone, where the temperature difference between flue gases and fluid conduit system is very high, the transfer of heat from the steel pipes to the steam is much better than that from the flue gases to the steel pipes,

thereby keeping the steel pipes close to the temperature of the exiting vapor rather than a higher temperature. This is desirable since the steam exits at l050 F. and the temperature should preferably not exceed l,400 F. and certainly not exceed 1,600 F.

Since the density of the fluid increases with decreasing temperature, the volume of fluid flowing through the vapor phase zone is greater than that flowing through the boiler, which in turn is greater than that flowing through the liquid phase zone. The circular arrangement takes care ofthis to a certain extent, but in addition it will be necessary to have a greater number of tubes per pass in the outer zones than in the inner zones. This results in relatively few tubes in the liquid phase zone so that extended surfaces are provided for heat exchange. The relatively many tubes in the vapor phase zone contributes to permit the elimination of extended additional surfaces for heat exchange. It should also be remembered that the fluid traverses the boiler zone but once while it traverses the liquid phase zone and the vapor phase zone several times.

In the particular heater design shown in FIGS. l-3, for example, there are 36 parallel paths in the liquid zone, 180 parallel paths in the boiler zone, 160 parallel paths in the initial part of the vapor zone, and 288 parallel paths in the final vapor zone. Thus, there is approximately a 10 to 1 increase in the number of paths between input and output, corresponding to a density ratio between incoming liquid and exiting vapor of 10 to l. This ratio is small relative to conventional boilers,

' wherein the ratio could be of the order of 200 to l. A contributing factor to the small ratio is the fact that the pressure and temperature in the boiler zone are close to the critical pressure and temperature of the fluid.

In a representative calculated compact fluid heater, in the liquid phase zone the water could rise for example from 200 F. to 300 F. in the innermost layer; in the second innermost layer the water could rise from 300 to 450 F.; and in the third innermost layer the water could rise from 450 to 636 F. During its passage through the boiler, the temperature of the fluid would remain at 636 F. In the first layer of the vapor phase zone, the steam would be superheated from 636 to 700' F., and in the two outermost layers connected in parallel, the superheated steam would rise from 700 F. to l,050 F., which would be the exit temperature.

The pressure difference between incoming liquid and outgoing vapor is small, the liquid entering at 2,050 psi. and the vapor exiting at 2,000 psi. The velocity of the fluid through the heater is about constant at about 60 feet per second. The transfer of heat from the flue gases to the fluid is about 460 B.t.u. per pound in the liquid zone, 5l0 B.t.u. per pound in the boiler zone, and 375 B.t.u. per pound in the vapor zone.

Temperature sensors may be provided to keep track of the temperature at various points in the circuit, and appropriate values can be obtained at various points in the circuit in response to the thermocouple signals that may alter the fluid flow or combustion rate in the manner desired.

The fluid heated is not limited to water, and such fluids as carbon dioxide may be preferred in space environment.

Having thus described the principles of the invention together with an illustrative embodiment thereof it is to be'understood that although specific terms are employed, they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims:

lclaim:

l. A compact fluid heater comprising a central flue; a conduit system including a multiplicity of conduits surrounding said flue parallel to the axis thereof and arranged in a plurality of concentric arrays which in turn are grouped into three concentric zones; means for introducing fluid under pressure into the innermost array of said conduit system, said conduit system including connection between said conduits so that, under said pressure, said fluid moves through said array to the outer periphery thereof through a radially outwardly progressive sequence of paths each of which is predominantly parallel to said axis, forced convection means for forcing gas through said array from the outer periphery thereof into said central flue for axial exit therefrom whereby the movement of said gas through said conduit system is in cross counter flow with respect to the movement of said fluid through said conduit system; and means for heating said gas prior to entry through said array to a temperature sufficient to cause said fluid to boil in the middle zone.

2. A, compact fluid heater in accordance with claim 1 wherein extended surfaces are provided in the innermost and middle zones, said extended surfaces being mechanically connected within the innermost zone to conduits included within the same array,.there being no connection of any such extended surface in such innermost zone to conduits in more than one array, the extended surfaces in said middle zone extending through the entire zone and therefore interconnecting conduits in different arrays, the connection between adjacent arrays in the innermost zone being solely by means of the return conduits or elbows and the mechanical connection between adjacent arrays in the outermost zone being solely by means of headers at the ends, whereby thermal stresses are avoided throughout the heater since adjacent arrays of conduits are mechanically separated in all areas where there is a substantial temperature difference between arrays.

3. A compact fluid heater in accordance with claim 1 wherein the gas flow is by forced draft and .is exclusively radially inward while the fluid flow movement in the radial direction is outward in cross counter flow, where varying the number of. tubes in arrays, as is now possible in the straight tube and header construction as opposed to helical tube coil convection with a pressure source. construction, compensates for the expansion of the fluid as it A Compaq fl id healer in accordance with daim and comraeuon gas as coded wherein boiling takes place close to the critical pressure and 4. Apparatus in accordance with claim I wherein the necessary heat is provided by burning a fuel in the gaseous phase to 5 allow complete combustion, that renders a blue nonluminous flame and where heat is transferred to the heater by forced temperature of the fluid thus permitting not more than a 30 to 1 density ratio between incoming liquid and outgoing vapor. 

